Unable to install molecular addon in blender. cmolcore is undefined
The molecular addon created by Pyroevil has not been updated for several years.

You can find a couple of forks of this addon, the one by scorpion81 has been updated to work with 2.80 and also has a branch for 2.79. There is another fork by PavelBlend that has more recent changes. Both of these offer some pre-built copies of the addon from their releases page. If you get a git snpahot of the addon then you will need to compile the addon to get it working, you will need cython and a c/c++ compiler for that.

If you have no luck with these forks, you might want to report an issue to one of these projects on github to see if they can help you.